1. Field of the Invention
The present invention is directed to optical systems for projection displays and more particularly to a high resolution improved optical system employing reflection light valves.
2. Discussion of the Prior Art
The development of liquid crystal flat panel displays has led to interest in using such panels as transmission light valves for a color projection display for data or video applications. A number of such systems are commercially available. However, in order to prepare light valves with a higher number of pixels as required for high resolution applications, the liquid crystal panel becomes large. It is difficult to produce panels with very small pixels since the electronic circuit structure needed for the operation of the panel obscures the light passage through the pixels to an intolerable degree. Instead, there is interest in light valves consisting of an array of pixels designed to operate in reflection instead of transmission. In reflection mode, it becomes possible to fabricate the mirror structure directly above the electronic circuitry. This mode allows smaller pixel areas without the obstruction of light passage caused by the circuitry, thus allowing maximum light throughput. This solves the pixel size problem, but introduces complexity into the optical system required to project an image of one or several cells onto a screen.
A major class of liquid crystal light valves operates in a birefringent mode. Liquid crystal light valves are one known type of a spatial light modulator. The operating principle is based on rotation of polarization of the incoming light for each pixel. An image is formed in a transmission liquid crystal cell when the cell is placed between two polarizers. In a reflection mode of operation, the challenge is to direct linearly polarized light onto the light valves and to form an image only using the light reflected with rotated polarization. The reflective light valve mirrors will reflect both polarizations of light. Light of the original polarization must be selectively removed from the reflected light beam and not be allowed to reach the screen.
U.S. Pat. No. 4,687,301 to Ledebuhr discloses a liquid crystal light valve projection system that includes a beam polarizer, a color separating assembly that comprises two color separators enclosed in a liquid filled enclosure, and liquid crystal reflection light valves for reflecting the separated colored light back into the separating assembly to be combined and directed into a projection lens. One of the drawbacks of this system is that portions of the source image are lost or significant portions of the illumination light are lost because of differences between the shape of the light introduced into the system and the shape of the light valves. Furthermore, typical illumination schemes usually produce non-uniform illumination on the light valves resulting in poor intensity uniformity and poor color uniformity in the projected image. Other drawbacks include a lack of polarization control and poor contrast. Additional problems include stray light reflected back from the projection lens and adverse effects of room light. Thus there is a need for an efficient optical system for achieving a full color high resolution display using several reflective birefringent light valves.